1. Field of the Invention
The present invention relates to a load drive circuit for switching on/off of a load to be driven.
2. Description of the Related Art
An automobile includes various in-vehicle loads (such as those engine-related, electric-related, information-related). In particular, recent developments in electronics have allowed various electronic units or the like to be loaded in an automobile as an in-vehicle load.
Furthermore, conventional various overcurrent protection has been provided by setting up a fuse 4 as shown in FIG. 2 on a current path 3 that connects a load 1 with a power source 2 (conventional technique 1). In FIG. 2, reference numeral 5 denotes a mechanical relay.
However, the use of the fuse 4 as shown above for overcurrent protection requires, whenever this fuse 4 fails, another operation in which the fuse is exchanged with a new one. Additionally, in general, there have been used a unit-type fuse box including a plurality of fuses 4. However, this fuse box has a large volume and occupies more space, thus reducing space for other in-vehicle electrical components. Such a fuse box is also limited in its mounting position because the fuse 4 must be exchanged with a new one.
In view of the above, there has been another kind of load drive circuit using a semiconductor relay instead of the fuse box.
Specifically, the semiconductor relay is used in two ways as shown below.
One way is that an overcurrent is detected by a shunt resistance or a sense MOS-FET or the like and the detected overcurrent is determined by a microcomputer or an external circuit (conventional technique 2). In this case, rush current is taken care by changing the reference voltage of the external circuit or by a software program of the microcomputer.
The other way is that a self protection type Intelligent Power Device (IPD) 6 as shown in FIG. 3 is used that has a current detection function and a determination function (conventional technique 3).
The IPD 6 of this conventional technique 3 has a self protection type overcurrent protection function as shown in FIG. 4 that works, when a load drive circuit itself has therein an overcurrent or overtemperature, to detect the overcurrent or overtemperature to block the current. In this case, the fuse 4 in FIG. 3 may be eliminated.
When the on/off switching of driving to the load 11 is provided by a drive switching element (a first switching element) 12 consisting of FET, this IPD 6 as shown in FIG. 4 controls the ON/OFF state of this drive switching element 12 and provides current protection of the drive switching element 12.
Specifically, when an operator uses the operation switch 13 to provide an on/off switching operation, the ON/OFF state of the operation switch 13 is detected by the input circuit 15. When the input circuit 15 detects the ON state of the operation switch 13, the protection circuit 21 and the gate drive circuit 23 are provided with the power source (+B) 19, thereby starting the operation.
The gate drive circuit 23 is called a charge pump that uses, in order to allow the gate of the drive switching element 12 to have a higher electric potential than that of the source, an N channel FET and a oscillation capacitor or the like to increase the voltage of the power source (+B) 19 (for example, to double the voltage).
The current limitation circuit 25 determines whether the voltage decline between the drain and the source of the drive switching element 12 exceeds a predetermined threshold value or not. When the voltage decline between the drain and the source of the drive switching element 12 exceeds the predetermined threshold value, the current limitation circuit 25 then causes the gate and the source to have therebetween a short circuit to reduce the input voltage to the gate, thereby reducing the current flowing in the drive switching element 12.
This IPD 6 has a overcurrent detection circuit 29 for detecting an overcurrent to report information to the protection circuit 21 and an overheat detection circuit 31 for detecting an overtemperature to report information to the protection circuit 21. When the overcurrent detection circuit 29 detects an overcurrent or when the overheat detection circuit 31 detects an overtemperature, then the protection circuit 21 blocks or intermittently stops via the gate drive circuit 23 the supply of the gate voltage to the drive switching element 12, thereby adjusting the current and temperature.
These conventional techniques 2 and 3 significantly reduce the conventional number of times for which the fuse 4 is exchanged with a new one, thus reducing manhours accordingly. These conventional techniques 2 and 3 can also eliminate the fuse box itself and thus provide more mounting space for other components.
For reference, JP-A-2000-312433 discloses a related art.
By the way, the load drive circuit using the conventional IPD 6 had a problem in that, even when the input terminal is in an OFF state and the drive switching element 12 is in an OFF state, power is always supplied to the circuit sections 21, 23, 25, 29, and 31 in the load drive circuit to be in an active status, thus these circuit sections 21, 23, 25, 29, and 31 always have therein current. This current causes a dark current, thus causing a device such as an in-vehicle electrical component driven by a battery to have a reduced battery life.